1. Field of the Invention
This invention relates to a III group nitride system compound semiconductor light emitting element and, particularly, to a III group nitride system compound semiconductor light emitting element that includes a quantum well structure and emits light in the range of ultraviolet.
2. Description of the Related Art
Japanese patent application laid-open No.2000-294884 (prior art 1) discloses a light emitting element that has a well layer including Al with a large bandgap energy to offer a short-wavelength emission. In the light emitting element, both of well layer and barrier layer include Al and the Al composition of barrier layer is greater than that of well layer. Thereby, the difference in lattice constant between well layer and barrier layer is reduced to suppress piezo-electric field to achieve a high emission efficiency. Well layer in embodiment of prior art 1 is of three element mixed crystal, AlGaN.
On the other hand, “300 nm band high intensity ultraviolet LED using four element mixed crystal InAlGaN”, Monthly Display 2001 August Separate Volume, August, 2001 (prior art 2) discloses a high-output short-wavelength light emitting element that uses four element mixed crystal, AlGaInN as III group nitride system compound semiconductor. Prior art 2, FIG. 8, shows a light emitting element that active layer of AlGaInN is sandwiched by AlGaN layers and the Al composition of the former layer is greater than that of the latter layers. However, the light emitting element in FIG. 8 includes no quantum well structure.
In the light emitting element of prior art 1, the difference in lattice constant between well layer and barrier layer is, as described above, reduced by introducing Al to both of well layer and barrier layer. However, the light emitting element of prior art 1 has the following problem.
In prior art 1, the Al composition of well layer is smaller than that of barrier layer. According as the Al composition I of well layer is increased to give a shorter wavelength emission, the Al composition of barrier layer has to be increased to exceed that of well layer. In this case, the barrier layer must be subject to an increase in crystalline defect and an increase in resistivity. As a result, the emission output thereof will be reduced.